Synthetic retinoids effective against MRSA persister cells

Share

Copy the link

Discovery of new antibiotics and elucidation of their mechanism often require interdisciplinary collaboration. We would like to share our story about a successful collaboration with a group of clinicians, geneticists, physicists, engineers, and chemists from Brown, Harvard, and Emory University.

As we are part of an infectious diseases division, we are completed by the frequent relapses and the prolonged treatment associated with Staphylococcus aureus infections. This motivated us to study not only the ability of S. aureus to develop antibiotic-resistance, but also its ability to shift to non-growing dormant subpopulation of “persisters”. Indeed, S. aureus persisters formed in biofilms are known as a major reason for high relapse rates of endocarditis and osteomyelitis. Therefore, new classes of antibiotics are urgently required to treat these complex S. aureus infections.

To identify the new antibiotics effective against MRSA, we conducted a high-throughput screening project in collaboration with Frederick M. Ausubel. The major advantage of the C. elegans-based screening method is to identify anti-infectives while simultaneously excluding compounds that are toxic to the worm or have no in vivo efficacy. Using this screening methodology, we screened ~82,000 small molecules and identified 185 compounds that rescue C. elegans from MRSA infections.

Among hit compounds, we found that two synthetic retinoids (vitamin A analogues), CD437 and CD1530 that are formerly known bioactive compounds and share structural similarity to each other. Interestingly, their analogue, adapalene (a FDA-approved synthetic retinoid acne drug) was not identified as a hit. We found that CD437 and CD1530 kill both growing and persistent MRSA by inducing rapid membrane permeabilization without detectable resistance development. By comparing CD437 and CD1530 with adapalene, two polar branch groups, we easily noted that carboxylic and a hydroxyl moieties play key roles in antimicrobial activity and membrane-permeability.

To understand the mode of action of these retinoids, we collaborated with geneticists, biophysicists and engineering scientists. First, Daria Van Tyne and Michael M. Gilmore analyzed the whole genome sequences of S. aureus mutants exhibiting modest retinoid-resistance and identified the mutated genes resulting in modest retinoid resistance. Interestingly, the identified mutated genes were related to bacterial membrane physiology. Based on this results, we hypothesized that our synthetic retinoids target S. aureus membranes.

In 2013, our lab moved from Harvard Medical School to Warrant Alpert Medical School of Brown University. We were able to continue to meet wonderful collaborators in this new place. To confirm our hypothesis, we collaborated with Brown membrane biophysicists, Petia Vlahovska (Currently Northwestern University) and her postdoc Nico Fricke who are experts on artificial lipid bilayers such as giant unilamellar vesicles to mimic bacterial lipid bilayers. Using the giant unilamellar vesicles they made, we were able to confirm that CD437 and CD1530, but not adapalene, interact with and disrupt lipid bilayers. We then questioned how our reitniods interact with lipid bilayers at molecular levels. To address this question, we reached out to Brown engineering scientists, Huajian Gao and his postdoc Wenpeng Zhu, experts on computational simulations of cell-nanomaterials interactions. By using molecular dynamic simulations, we found that our retinoids persistently bind to hydrophilic heads via two polar branch groups and then penetrate the bilayers using interactions between retinoids’ hydrophobic backbone and lipid tails. As a result, the retinoids embedded into outer membrane leaflet, causing substantial disruption of the membrane lipid bilayers.

To further confirm the functionality of each chemical groups of retinoids and assess the possibility of further optimization the expertise of medicinal chemists was needed. Coincidentally, Bill Wuest from Temple University (currently Emory University) was invited to Brown and presented his work. We discussed our results with Bill, and he was willing to collaborate with us. Bill and his graduate students, Andrew M. Steele and Colleen E. Keohane synthesized 16 analogues. We were able not only to confirm our simulation results and but also to synthesize ‘analogue 2’ that exhibits significantly reduced cytotoxicity while retaining antimicrobial activity (another lesson here is to never miss a seminar).

In addition to the scientific experiences through this process, we truly enjoyed working with scientists from 7 different countries (South Korea, China, United States, Germany, India, Bulgaria, and Greece). In addition to new compounds, scientific discovery can provide a social paradigm how different cultural and disciplinary backgrounds can synergize effectively.

In general, the enteric microbiota composition is relatively stable due to the ongoing competition of bacterial members for space and nutrients. Newly arriving bacteria hardly find an empty niche and sufficient nutrients to thrive and colonize. Shortly after birth, however, this situation is markedly different. The neonate is born sterile and newly incoming bacteria can easily find a place and nutrients to stay and colonize the neonate's intestinal mucosa. Notably, it is generally thought that this process is mainly driven by exposure to bacteria derived e.g. from the mother of the environment.
But is that really true? If only the environment determines the microbiota composition couldn't that go terribly wrong? Shouldn't we expect that host factors influence the emerging microbiota ensuring a beneficial bacterial composition?

This community is not edited and does not necessarily reflect the views of Nature Research. Nature Research makes no representations, warranties or guarantees, whether express or implied, that the content on this community is accurate, complete or up to date, and to the fullest extent permitted by law all liability is excluded.

Please sign in or register for FREE

Sign in to Nature Research Microbiology Community

Register to Nature Research Microbiology Community

The Nature Research Microbiology Community provides a forum for the sharing and discussion of ideas and opinions about microbiology. Through posts, discussion, image and video content, the community space can be used by members to communicate with each other, and with editors, about topics ranging from the science itself through to policy, society and day to day life. It is also a place to learn more about the activities of Nature Microbiology's editors and the policies and practices of the journal.